Integrated circuit production relies on the use of photolithographic processes to define the active elements and interconnecting structures on microelectronic devices. Until recently, g-line (436 nm) and I-line (365 nm) wavelengths of light have been used for the bulk of microlithographic applications. However, in order to achieve smaller dimensions of resolution, the wavelength of light used for microlithography in semiconductor manufacturing has been reduced into the deep UV regions of 254 nm and 193 nm. As the patterns and wavelengths become finer, the materials properties of the photoresists used for pattern delineation have become more and more demanding. In particular, requirements of sensitivity, transparency, aesthetics of the image produced, and the selectivity of the resists to etch conditions for pattern transfer become more and more strenuous. The problem with using deep UV wavelengths is that resists used at the higher wavelengths were too absorbent and insensitive. Because of this, the traditional lithographic materials, such as novolaks, diazonaphthoquinones, etc., are unsuitable platforms for ultra large-scale integration (ULSI) manufacture and beyond. Thus, in order to utilize deep UV light wavelengths, new resist materials with low optical absorption and enhanced sensitivities were needed along with additional lithographic techniques.
Advanced photoresists usually employ a technique called chemical amplification in which an acid generated by photolysis catalyzes a solubility switch from alkali insoluble to alkali soluble by removal of an acid sensitive group protecting an alkali-solubilizing moiety. The principle of chemical amplification as a basis for photoresist operation has been known for some years (see U.S. Pat. No. 4,491,628). Most chemically amplified resists have been designed around the use of acid sensitive carboxylic esters or acid sensitive hydroxystyrene derivatives.
However, chemically amplified resist systems have many shortcomings. One problem is standing wave effects, which occur when monochromatic is reflected off the surface of a reflective substrate during exposure. The formation of standing waves in the resist reduces resolution and causes linewidth variations. For example, standing waves in a positive resist have a tendency to result in a foot at the resist/substrate interface reducing the resolution of the resist.
In addition, chemically amplified resist profiles and resolution may change due to substrate poisoning. Particularly, this effect occurs when the substrate has a nitride layer. It is believed that residual N—H bonds in the nitride film deactivates the acid at the nitride/resist interface. For a positive resist, this results in an insoluble area, and either resist scumming, or a foot at the resist/substrate interface.
Furthermore, lithographic aspect ratios require the chemically amplified resist layer be thin, e.g., about 0.5 μm or lower, to print sub 0.18 μm features. This in turn requires the resist to have excellent plasma etch resistance such that resist image features can be transferred down into the underlying substrate. However, in order to decrease absorbance of the chemically amplified resist, aromatic groups, such as those in novolaks had to be removed, which in turn decreased the etch resistance.
The most common type of photoresists are called “single layer” photoresists in which the photoresist has both the function of imaging and plasma etch resistance. Single layer resists use anti-reflective coatings to minimize standing wave problems, which also assists in decreasing problems with substrate poisoning. However, there are distinct performance tradeoffs between resist absorbance, image profiles, resolution and substrate plasma etch resistance which are not optimum for all semiconductor processes.
Utilizing an underlayer or undercoat film that is placed on the substrate before the chemical amplified resist film is applied can reduce the above-mentioned problems and break the performance tradeoffs described above. The undercoat absorbs most of the deep UV light attenuating standing wave effects. In addition, the undercoat prevents deactivation of the acid catalyst at the resist/substrate interface. Furthermore, the undercoat layer can contain some aromatic groups to provide substrate etch resistance.
In the typical bilayer resist process, the undercoat layer is applied on the substrate. The chemically amplified resist is then applied on the undercoat layer, exposed to deep UV light and developed to form images in the chemically amplified resist topcoat. The bilayer resist system is then placed in an oxygen plasma etch environment to etch the undercoat in the areas where the chemically amplified resist has been removed by the development. The chemically amplified resist in a bilayer system typically contains silicon and is thus able to withstand oxygen plasma etching by converting the silicon to silicon dioxide, that then withstands the etch process. After the bottom layer is etched, the resist system can be used for subsequent processing such as non-oxygen plasma etch chemistry that removes the underlying substrate.
Even though the undercoat attenuates standing waves and substrate poisoning, it poses other problems. First, some undercoat layers are soluble to the chemical amplified resist solvent component. If there is intermixing between the top and undercoat layers, the resolution and sensitivity of the top resist layer will be detrimentally affected.
In addition, if there is a large difference in the index of refraction between the chemical amplified resist and the undercoat layer, light will reflect off the undercoat layer causing standing wave effects in the resist. Thus, the real portion “n” of the index of refraction of the two layers must be made to essentially match or to have their differences minimized, and the imaginary portion “k” of the index of refraction of the two layers must be optimized to minimize reflectivity effects.
Another problem with undercoating layers is that they are sometimes too absorbent because of incorporation of aromatic groups. Some semiconductor manufacturing deep UV exposure tools utilize the same wavelength of light to both expose the resist and to align the exposure mask to the layer below the resist. If the undercoat layer is too absorbent, the reflected light needed for alignment is too attenuated to be useful. However, if the undercoat layer is not absorbent enough, standing waves may occur. A formulator must balance these competing objectives.
In addition, some undercoats have very poor plasma etch resistance to plasma chemistry. The etch resistance of the undercoat should be comparable to the etch rate of novolak resins in order to be commercially viable.
Furthermore, some undercoat layers require UV exposure in order to form cross-links before the radiation sensitive resist topcoat layer can be applied. The problem with UV cross-linking undercoat layers is that they require long exposure times to form sufficient cross-links. The long exposure times severely constrain throughput and add to the cost of producing integrated circuits. The UV tools also do not provide uniform exposure so that some areas of the undercoat layer may be cross-linked more than other areas of the undercoat layer. In addition, UV cross-linking exposure tools are very expensive and are not included in most resist coating tools because of expense and space limitations.
Some undercoat layers are cross-linked by heating. However, the problem with some of these undercoat layers is that they require high curing temperatures and long curing times before the top layer can be applied. In order to be commercially useful, undercoat layers should be curable at temperatures below 250° C. and for a time less than 180 seconds. After curing, the undercoat should have a high glass transition temperature to withstand subsequent high temperature processing and not intermix with the resist layer.
Even at temperatures below 250° C. sublimation of small a mounts of the undercoated formulation (e.g. TAGs, oligomers from the polymer, etc.) or products (e.g. acids, alcohols, water) from the thermal curing frequently occur. This can result in contamination of the equipment, which requires more frequent cleaning or replacement of equipment to avoid loss of product yield and other measures. This results in increased costs and is undesirable.
In certain applications, it is desirable for the undercoat to planarize the surface of the substrate. However, this may be difficult to accomplish with undercoats undergoing thermal crosslinking. As the temperature rises and the number of crosslinks increase, the glass transition temperature of the film increases. This makes it more difficult for the film to flow and planarize the substrate. Thus it is desirable to use as low a molecular weight material as possible to improve the planarization. However, use of lower molecular weight polymers in the undercoat can adversely impact the lithographic performance of the photoresist imaging process.
In addition to the requirements for no intermixing, no sublimation, good substrate plasma etch resistance, good planarization properties, and optical properties appropriately complementing the photoresist coated over the undercoat, the undercoat must be compatible with at least one edge bead remover acceptable to the semiconductor industry as well as give excellent compatibility with the photoresist coated over the undercoat so that lithographic performance (e.g. photospeed, wall profiles, depth of focus, adhesion, etc.) are not adversely impacted. Solutions for these issues individually frequently adversely impact the performance in other areas. The present invention is directed to thermally curable polymer compositions with decreased sublimation tendencies in which lower molecular weight polymers may be employed without major adverse impacts in other performance areas.